Dr V. S. N. Vithanaarachchi
Senior Lecturer, Consultant Orthodontist, Faculty of Dental Sciences, University of Peradeniya,
The development of the transverse discrepancy has a multifactorial etiology with dental crowding and crossbite being frequent presentations. Expansion of the arches has been increasingly performed in orthodontics to gain better occlusal relationship. Widening of the arches can be gained with orthodontic, passive, and orthopedic expansion. There is a controversy regarding ideal timing to perform arch expansion and most of the researches have highlighted pronounce transverse skeletal changes during the pubertal growth spurt. Clinical indication of maxillary expansion is more frequently practiced by the clinicians. Rapid maxillary expansion, slow expansion, and surgically assisted rapid palatal expansion are the main expansion modalities. Heavy and rapid forces are applied to the teeth via rapid maxillary expansion and orthopedic expansion is significant. The slow expansion delivers a constant physiologic force until the required expansion is obtained. As the age advances, the effect of maxillary expansion diminishes and surgically assisted expansion is considered in widening the arch. Mandibular expansion is much difficult to perform compared to the maxillary expansion due to compact structure of the bone. Stability of the arch expansion is still being debated and most of the researchers have identified that maxillary expansion is relatively stable, compared to the mandibular expansion. Keyword: Maxillary expansion, rapid maxillary expansion, slow maxillary expansion, surgically assisted maxillary expansion, mandibular expansion
Transverse growth is found to achieve near completion by late adolescence, but sagittal and vertical growth continue into adulthood1. However, recent researches have argued and shows evidence of overlap in three dimensional changes of the jaw growth. Growth of the transverse dimensions such as cranial and interjugal width end much before antero-posterior and vertical growth and interzygomatic and intergonial widths continue to increase into adulthood. Researchers have identified that the development of transverse discrepancies is due to multifactorial etiology and eliminating the etiological factors become difficult. Some of the influencing factors for the transverse growth can be listed as genetic, age, gender, race and ethnicity, growth patterns, habits, muscles, skeletal differentiation, malocclusions and orthodontic intervention1. Transverse narrowing of dental arches are common among individuals and dental crowding and crossbite are common clinical presentations of them. Further, most of the dental irregularities arise when an insufficient arch perimeter is existing between the 1st permanent molars which don’t not permit proper alignment of erupting permanent dentition. Expansion of arches has been increasingly performed in orthodontics to gain better occlusal relationship.
Expansion can be divided into three groups namely orthodontic, passive and orthopedic. Orthodontic expansion of dental arches results in lateral movement of buccal segment of dental arches and introduce lateral tipping of posterior teeth3. Passive expansion is achieved by use of lip bumper, vestibular shied of Frankel appliance while widen the dental arches by shielding the occlusion from the forces from buccal and labial musculature. Orthopedic expansion is considered as skeletal and separates the mid palatal suture using rapid palatal expansion or other form of treatment mechanics.Decision making on expansion or extraction as a space gaining method requires a thorough assessment of the patient profile, inclination of incisors, severity and growth potential of the patient. All in all this article aims to review arch expansion mechanics used in orthodontic practice.
The narrow maxilla has been recognized for thousands of years by “Hippocrates”. The first published work originated in United States with Angell in (1860) who placed a screw appliance between maxillary premolars of a girl ageed 14.5 years and widened her arch one quarter of an inch in two weeks. To many in the field of orthodontics and Rhinology, the early 1900’s are known to be the “maxillary expansion years”. In the year 1889, the president of the American Dental Association J. H. McQyillen had a protestation against Angell. Such protestations were responsible for Angell’s future silence. Lundstrom and later Brodie and associates also challenged the expansion. Maxillary arch expansion has been practiced for more than a century to address the maxillary transverse deficiency and the earliest common cited report is that of E.C. Angell published in Dental Cosmos in 1860. The rapid maxillary expander was first described by Emerson Angel in 1860 and later repopularized by Hass. The bonded rapid platal expander was first described by Cohen and Silverman in 19735. A number of slow expansion techniques were employed by early dental practitioners like Fauchard (1728) Bourdet (1757), Fox (1803), Delabarre (1819), Robinson (1846) and White (1859). Coffin spring was introduced by the Walter Coffin in 1875. In 1902 Pierre Robin constructed a split plate which was incorporated with a screw. Repulsive magnetic forces for maxillary expansion were first introduced by Vardemon in 1987.
There is a controversy regarding the ideal time to perform arch expansion. Some researchers have shown that better results can be obtained when performed early than permanent dentition7. However, some researchers have shown that maxillary expansion may also be successfully done in older adolescents and adults. Available information related to the ideal time for treatment of maxillary transverse deficiency with orthopedic devices mainly related to the studies done with growth and maturation of intermaxiallry sutural system. Melsen conducted a land mark study on this with autopsy material with histological assessment of maturation of the mid – palatal suture at different developmental stages9. According to his study, in the “infantile” stage (up to 10 years of age), the suture was broad and smooth. Whereas, in the “juvenile” stage (from 10 – 13 years) it had changed into a more typical squamous suture with overlapping sections.
During the “ adolescent “ stage (13 – 14 years of age ) the sutures showed wavier appearance and increased interdigitation. They have observed same histological presentation of adults with their study conducted in 1982, and identified that more synostoses and numerous bony bridges formation across the mid palatal suture10. These findings were further confirmed with the results identified by the Wertz and Dreskin11 highlighting that more stable orthopedic changes in patients under the age of 12 years. Further, Bjork implant studies have confirmed that the transverse growth pattern of the maxilla also follows the distance and velocity curves similar to those of body height12,13. Bacetti also confirmed that rapid maxillary expansion treatment before the peak in skeletal growth velocity is able to induce more pronounced transverse craniofacial changes.
The clinical conditions where maxillary expansion is indicated in orthodontic practice are cross bites, distal molar movement, functional appliance treatment, arch coordination in orthognathic surgery, to facilitate maxillary protraction and correction of mild crowding. Correction of the transverse discrepancy of maxilla usually requires expansion of the palate by a combination of orthopedic and orthodontic tooth movements. Three expansion modalities are practiced by the clinicians namely rapid maxillary expansion (RME), slow maxillary expansion and surgically assisted maxillary expansion. Each of these expansion methods have advantages as well as disadvantages and many controversies exist regarding their usage.
Further, conventional radiographs, such as cephalometric and panoramic radiographs, are not appropriate for examining buccal bone or periodontal changes during and following completion of RME therapy. As these investigation techniques are based on a two-dimensional representation of a three-dimensional object it does not allow the orthodontist to evaluate buccal bone widths or to measure transverse changes associated with maxillary expansion.
The amount of the orthopedic and orthodontic changes depends mainly on the patient’s age. Usually palatal growth is nearly completed by the age of six years and increases the inter- digitalization of mid palatal sutures after puberty. Therefore, rapid palatal expander appliances require frequent activations to generate heavy forces such as 2 – 5 kg per quarter – turn with accumulated loads of more than 9 kg. Furthermore, researchers have identified the difference of oxidation status and interleukin – 1 β during RME treatment, which could be attributed to orthopedic effect of the heavy force on maxillary and minimal orthodontic forces on teeth. There are few disadvantages with the use of rapid maxillary expanders such as discomfort due to heavy forces, traumatic separation of mid palatal suture, inability to correct rotated molars, increase vertical height and bite opening, chance of relapse and requirement of a bystander for the activation of the appliance. The recently introduced Keles keyless expander achieve maxillary expansion with minimal patient corporation as it does not require a key for activation. Basically RMEs can be divided into two main groups as banded and bonded appliances.
The banded appliances are more hygienic than bonded device as there is no palatal coverage. The banded RME can be further divided into two types as follows
1. Tooth borne RME
2. Tooth and tissue borne RME
Tooth borne RME consist of only bands and wires without any acrylic coverage. HYRAX expander is a common tooth borne appliance and which was introduced by William Biederman in 1968. The Hyrax expander is a non spring loaded jackscrew with a wire frame. Its main advantages are the absence of irritation to the palatal mucosa and is easy to keep clean. Each activation of the screw produces approximately 0.2mm of lateral expansion and a maximum of 13mm can be achieved.
Isaacson expander also a tooth borne RME and it makes use of a spring loaded screw called Minne expander (developed by the University of Minnestota, Dental School). The Minne expander is a heavily calibrated coil spring, expanded by turning a nut to compress the coil and two metal flanges perpendicular to the coil are soldered to the bands on abutment teeth. Further, the Minne expander may continue to exert expansion forces after completion of the expansion unless it is partly deactivated.
Tooth and tissue borne RME consist of an expansion screw with acrylic abutting on alveolar ridges. It has few advantages over tooth borne RME such as it produces more parallel expansion, less relapse, greater nasal cavity and apical base gain while creating more mobility of the maxilla. However, tooth and tissue borne RME tend to develop more soft tissue irritation. Hass is one of the tooth and tissue borne RME indicated for the rapid expansion with immediate mid palatal suture separation by disruption of sutural connective tissue. As described by the Hass this is a rigid appliance designed for maximum dental anchorage and able to maximize the orthopedic effect with range of 3- 10 ponds. Derichsweiler appliance is an another tooth and tissue borne RME which is banded to the first premolar and molars consisting of a slit palatal acrylic with a screw.
Bonded rapid maxillary expanders are more similar to the banded expanders except for the method of attachment to the teeth. This appliance is made with an acrylic cap over the posterior teeth. The bonded appliance is increasingly popular among clinicians due to easy cementation in mixed dentition stage, few recall appointments, reduced posterior teeth tipping and less extrusion due to buccal capping. Furthermore it is useful in class II correction and provides bite block effect to facilitate the correction of anterior crossbite with occlusal clearance.
Several appliances have been used by the clinicians for the purpose of slow expansion such as coffin spring, magnets, W- arch, quedhelix, spring jet and NiTi expanders. Coffin spring consist of an omega loop of 1.25 mm thickness which is positioned in the region of mid palatal suture and free ends of the omega loop are embedded in acrylic covering of the removable appliance. The spring is activated by pulling the two sides apart manually by 2-3 mm and the appliance is capable of gaining more dento- alveolar expansion.
However, banded magnets are able to produce skeletal and dental expansion and the degree of expansion depends on the growth status of the patient. The main advantage of the magnets is that they apply force over a long period of time with a decreased risk of external root resorption. There are few disadvantages of this method, such as the magnet having tendency to be oxidized in the oral environment due to potential formation of corrosive products. As the magnets are quite bulky it should be adequately stabilized and should contain stout guide rods to prevent unwanted rotational movements.
The W- arch is a fixed device constructed of 36 mil steel wire soldered to the molar bands and it was originally used by the Ricketts and his colleagues in the management of cleft palate patients. The device is easily activated by opening the apices of the arch and it delivers adequate force levels when it is opened 3-4 mm wider than the passive width. The W- arch should be constructed with 1- 1.5mm palatal clearance to avoid soft tissue irritation.
The quadhelix appliance was described by Ricketts and it is a modification of the W – spring while incorporating four helices to the spring. This has improved the flexibility and range of activation compared to the W-arch.
The length of the palatal arms of the appliance can be altered depending on the extent of the arch expansion. Recently introduced new generation prefabricated quadhelix appliance with nickel titanium is more favorable in force delivery system due to its superelastic properties. The quadhelix appliance is useful in buccal tipping and skeletal expansion in a ratio of 6:1 in growing children. 400gm of force level can be delivered by activating the appliance by 8mm which is approximately equal to one molar width. Overcorrection is advisable with quadhelix appliance as relapse is inevitable and three months retention period is needed following active correction. A large range of action, better retention, orthopedic effect, differential expansion, habit breaker and possibility of incorporate fixed appliances are some of the benefits of the quadhelix appliance. However, molar tipping, bite opening and limited skeletal changes are some of the limitations of the appliance. The spring jet is also useful in slow maxillary expansion and it has a telescopic unit which is placed up to 5mm from the center of molar tubes transmitting forces close to the center of resistance of maxillary teeth. Applied force is varied from 240gm to 400gm depending on the mixed dentition and permanent dentition respectively. Activation of the appliance is performed by moving the lock screw horizontally along the telescopic tube and a ball stop on the transpalatal wire allows the spring to be compressed.
The NiTi expander was introduced by Wendell V and it generates optimal, constant expansion effect on the maxillary arch. Its central component is made by the thermally activated NiTi alloy and the rest is made of stainless steel. At room temperature, the expander is too stiff and hard to bend for insertion as its transition temperature is 94° F. Chilling may softens the central component which makes easy manipulation. Once the device is placed in the oral cavity, it stiffens and begins to return to its original shape with the oral temperature.
As the age advances, the effect of maxillary expansion diminishes and surgically assisted expansion is considered in widening the arch and to correct posterior cross bites more than 7mm. Two techniques have been practiced by the clinicians such as surgically assisted rapid palatal expansion (SARPE) and segmental maxillary surgery to expand the maxilla. The SARPE is more popular as a treatment option for maxillary transverse deficiency as it allows the clinician to gain maxillary expansion effectively in mature patients. Segmental maxillary surgery supports transverse expansion during Le Fort 1 osteotomy with additional surgical cut along the midpalatal suture. However, the relative inelasticity of the palatal muco-periostium may limits the degree of expansion. It is needed to move the roots of the maxillary central incisors apart with orthodontic treatment prior to surgery to prevent the damage to the roots. Maxillary expansion with surgery is benefitial to patients with sagittal and / or vertical maxillary discrepancies. However, changes in the maxilla due to expansion are accompanied by a spontaneous mandibular response which in turn increases the mandibular arch perimeter.
Mandibular arch crowding is frequently observed in patients with malocclusions. The extent of crowding will determine the space gaining method during orthodontic treatment. However, it is more difficult to induce much changes to the mandible compared to maxilla due to its compact structure, which makes it more resistant to orthodontics as well as for orthopedic forces. Patients with mandibular arch deficiencies have been successfully treated with arch expansion using Schwarz devices or functional devices like lip bumpers.These therapies are commonly recommended to the patients particularly with lingually tipped teeth and relatively stable results have been shown with younger patients.
Fidan has reported successful outcome with the usage of a Trom borne appliance with fixed orthodontic treatment and increase in transverse and sagittal dimensions of the mandibular arch. Trom bone appliance simulates the natural lingual pressure applied by the tongue, which represents the most natural method of arch development. This approach has shown excellent potential for adult treatment due to minimal interference with speech . The ortho- system is comprised of two different appliances the TrainerTM and Farrell Bent Wire System (BWS). These two appliances combinely produce arch expansion, development and treat myo functional habits both in maxilla and mandible simultaneously . They have shown that the Trainer combined with BWS produces higher amount of expansion and higher increase in arch perimeter.
An expected relapse in the amount of maxillary expansion has been reported by some authors as a result of the pressure delivered by the cheeks on the maxillary arch and the resistance to deformation of maxillary sutures and surrounding tissues to maxillary expansion.
Krebs reported with his implant study, that the patients treated with RME found that the dental arch width was maintained in fixed retention phase and once the fixed retention was discontinued, there was a substantial decrease in the dental arch width, which continued to decrease over a period of 4 -5 years. Further, Hass reported the long term stability of RME and concluded that 9-12mm of expansion of maxillary posterior teeth and 4-5mm of expansion of mandibular intercanine width was stable. Sandstrom reported that an increase in intermolar and intercanine width of the mandible on completion of treatment with moderate relapse in retention phase with a group of patients treated with RME. Davis and Kronman repoted to the literature that patients treated with RME with no treatment in the mandibular arch and found an increase in mandibular intermolar and inter canine widths, without any clinical significance. Adkins et. al 39 assessed the relationship of arch perimeter and transverse width change in the maxilla following removal of RME and they found that arch perimeter has increased approximately 0.7 times in relation to the 1st premolar region.
Nance reported that mandibular expansion is unstable and that inter canine width returns to pre treatment values over period of time1. However, Herberger reported that 68% of expansion due to increase in mandibular inter canine width after long period of retention40. Nevertheless, the stability of mandibular arch expansion is still being debated.
The overall expansion can be significantly influenced by the type of treatment, growth status of patient, patient cooperation and modes and length of retention. Most of the literature shows that the maxillary expansion is relatively stable, when compared with the mandibular expansion. When considering the clinical implications of arch expansion and effective planning of the treatment, it seems that further research is warranted in this area.
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